This
Viking Orbiter image shows Uranius Tholus, one of the smaller volcanos
in the Tharsis region of Mars. It is only 60 kilometers across and 3
kilometers higher than the surrounding plains. In comparison with Olympus
Mons (slide #11), the greater number of
impact craters near Uranius Tholus implies that it is substantially
older than Olympus Mons. One such crater in the top center of the image
has been flooded by lava from the surrounding plains. Because this crater
must have formed after the volcano but before the plains, the plains
must be younger than the volcano. (This is an example of using superposition
relationships to determine the relative age of a series of features
by determining which features lie on top of other features.) This area
is believed to be more than 3 billion years old.